
I have a long haired breed too, one of which I have bred from. Toward the end of a pregnancy, the bitches shed some of their tummy hair themselves in readiness, but I have shaved my girls tummy already - initially this was for scans although it's not always necessary, but with mine having full coats, I like to shave. It won't do any harm whichever way you go. My girl is 2 weeks away from whelping now so I will keep her shaved from now until puppies are born then leave it to grow back on it's own.
Hope that helps